Wind energy is increasingly recognized as a viable option for complementing and even replacing other types of energy sources such as for instance fossil fuels. In the early development of wind energy, the majority of wind turbines were constructed for operation at a constant speed. However, recently the trend is toward using variable-speed wind turbines to better capture available wind power. In most cases, wind turbine blade pitch angles can be adjusted to control the operation of the variable speed wind turbine.
When the electrical power, which is fed into an electricity network, is larger than the electrical power, which is extracted from the electricity network, the frequency of the whole electricity network will increase. This is based on the fact that the wind turbines, which are connected to the electricity network, cannot deliver all their generated power to the electricity network. As a consequence, the frequency of the produced alternating current (AC) of the generated electrical power will increase. Typically, such a behavior will be shown not only by one wind turbine but by a plurality of wind turbines of a wind turbine park. Therefore, a power imbalance, wherein the produced power is larger than the used power, may result in an overall increase of the frequency of the electricity network.
In order to avoid such frequency variations it is known to operate wind turbines with a reduced power set point such that the electrical power, which is fed into the electricity network, will be reduced and a power balance between the power being delivered to the electricity network and the power being removed from the electricity network can be achieved at least approximately. Such a reduction of the power set point may even be requested by legal regulations in order to guarantee a stable electricity network.
The reason why a wind turbine is operated with a reduced power set point is typically not in the sphere of influence of the operator of the wind turbine. Therefore, the operator of a wind turbine will typically receive a revenue, which is related to the available power production and not to the actual power production. Therefore, it is highly desirable for an operator of a wind turbine to always have a precise and a reliable knowledge about the available power production of its wind turbine(s). Of course, the available power production will strongly depend on the actual wind condition.
It is known to estimate the available power production of a wind turbine by measuring the wind speed for instance at the top of the wind turbine nacelle and, based on the measured wind speed, to calculate the available power production. The available power production may be taken from a power curve table for a wind turbine, wherein the graph in of the power curve indicates how large the electrical power production will be at different wind speeds. The power curve table is generated by a field measurement executed by e.g. the wind turbine producers and are standardized for each special type of wind turbine. This known procedure for estimating the available power production has the disadvantage that the results are typically not very precise due to an inaccurate measurement of the wind speed by the anemometer.